1. Field of the Invention
The present invention relates to a laser oscillator that performs optical excitation, and more particularly to a laser oscillator having an exciting light source such as an exciting semiconductor laser diode (LD) arranged at the side of the laser medium, in which when there is an non-uniform distribution of the excitation produced in the laser medium, uniformity of the distribution of the laser oscillator energy can be achieved in a plane perpendicular to the optical axis of the laser medium by using different optical paths for the light reciprocating in each direction in the laser medium.
2. Description of the Prior Art
Solid-state laser excitation light sources used in the prior art include flash lamps. Although flash lamps are able to uniformly excite laser rods, their low conversion efficiency is a drawback. This led to the appearance of diode-pumped solid-state lasers that use a semiconductor laser as the exciting light source, which resulted in a major improvement of the conversion efficiency, from electrical input to laser output. It also became possible to improve the efficiency by using a slab-shaped laser medium
It is well known that in order to generate a laser beam in which the beam section has a uniform energy density, the laser medium should be uniformly excited. In the case of a diode-pumped solid-state laser, the pumping laser beam emitted from one side of the laser rod makes a non-uniform excitation distribution in the rod, so the lasing transverse mode is raised to a higher-order mode, or the beam divergence angle is increased due to the deviation of the beam section from the round.
To avoid this, taking into account the non-uniformity in the excitation distribution produced in the rod by diode-side-pumping, uniform optical excitation distribution has been attained using summation of the optical paths between the ends of the laser resonator. For example, Reference 1 (L. E. Holder, C. Kennedy, L. Long and G. Dub: xe2x80x9cOne joule per Q-switched pulsed diode-pumped laser,xe2x80x9d IEEE J. Quantum Electron. vol. 28, no. 4, pp. 986-991, 1992) used an LD excited laser having the rod laser 21 with the rectangular section shown in FIG. 10 to form a zigzag optical path. As another example, Reference 2 (J. J. Kasinksi, W. Hughes, D. DiBiase, P. Bournes and R. Burnham, xe2x80x9cOne joule output from a diode-array-pumped Nd:YAG laser with side-pumped rod geometry,xe2x80x9d IEEE J. Quantum Electron., vol. 28, no. 4, pp 977-985, 1992) describes the configuration shown in FIG. 11 having a cylindrical laser rod 22, in which an excitation distribution close to a fundamental transverse mode intensity distribution was obtained and higher-order transverse-mode oscillation was suppressed by using LD excitation directed towards the center axis from a number of sides. In FIGS. 10 and 11, 23 is an end mirror, 24 are exciting LD""s and 25 is an output mirror.
Laser oscillators that use two opposing Porro prisms have already been reported. While there can be mis-alignment of oscillator components and skewing and the like, due to the high stability of the lasing mode these laser oscillators have been used for special applications. Such a laser is described in Reference 3 (A. Maitland and M. H. Dunn, Laser Physics (North-Holland Publishing Company Amsterdam-London 1969), Chap. 11, Sec. 4, pp. 305-309). Also, Reference 4 (M. K. Chun and E. A Teppo, xe2x80x9cLaser resonator, an electrooptically Q-switched Porro prism device,xe2x80x9d Applied Optics, vol. 15, no. 8, pp. 1942-1946, 1976) describes a pulse laser that uses a laser oscillator having a pair of opposing Porro prisms and a Q-switch.
Since the laser oscillators described in References 3 and 4 use lamp excitation, the excitation distribution in the laser rod is highly uniform and there is no need to even out the distribution. Therefore, there is no need to form the ridgelines of the two Porro prisms at any special angle other than parallel, 45 degrees or 90 degrees. The aim in setting these angles is mechanical stability of the resonator or to set the Q-switch. With these angles, during each round trip in the resonator, the optical path is not rotated, or is rotated 90 degrees or 180 degrees around the optical axis of the resonator. This rotation has not been noticed or been ignored in considering the laser modes so far, as well as in respect to the aim of effecting uniformity of the non-uniform excitation distribution.
However, the following problems are known to arise in the case of a laser oscillator used for optical excitation having a configuration in which the exciting light source is a semiconductor laser or the like disposed at the side of the laser medium. When a laser rod is used having a rectangular cross section, as shown in FIG. 10, and the excitation is both sides, the laser beam is internally reflected in a zigzag path and uniformity of the excitation distribution is achieved with respect to the laser beam. However, uniformity of the excitation distribution perpendicular thereto is not effected, so beam parameters such as beam diameter differ in the vertical and horizontal directions of the lasing beam section, resulting in a flat elliptical beam that readily produces higher-order modes.
In the case of a configuration using a cylindrical laser rod, as shown in FIG. 11, a complex structure has to be used to mount the LD""s around the laser rod. This complicates the process of assembling the LD""s and rod, increases the weight and degrades the mechanical strength. In addition, piping to carry the cooling fluid used to cool the LD""s and the laser rod has to be connected to the LD mounts without blocking the lasing beams, necessitating the use of complicated routing for the cooling pipes. This complicates maintenance and checking, increasing the risk of malfunction. When the LD""s and laser rod are cooled by thermal conduction rather than cooling fluid, the thermal conduction path is long and complicated. Moreover, LD""s are driven by a heavy electric current, and the wiring used to deliver the electric current is rendered complex by the need to connect up each LD on its mount.
An object of the present invention is to provide a laser oscillator that can produce a laser beam in which the beam section has a uniform energy density, even when the exciting light sources are disposed at one side of the laser medium.
To attain the above object, the present invention provides a laser oscillator comprising an optical path combining first Porro prism and second Porro prism, a laser medium on the optical path, an optical excitation laser diode that irradiates the laser medium with light, a polarizer disposed on the optical path, a first Porro prism disposed with the optical path coinciding with a point on a ridgeline thereof and with the ridgeline parallel or perpendicular to a plane of incidence of the polarizer, a second Porro prism disposed on the optical path at a second end thereof with the optical path coinciding with a point on a ridgeline thereof, an angle formed by the two ridgelines of the first and second Porro prisms being a predetermined angle other than zero degrees or 90 degrees.
The present invention can include an oscillator having a Q-switch disposed on the optical path. For example, the Q-switch is disposed between the second end of the laser medium and the second Porro prism, or an oscillator with a Q-switch disposed between the polarizer and the first Porro prism.
The laser diode used can be one disposed to irradiate the laser medium with light from a side of the optical path.
The oscillator can include one having a control circuit for controlling duration of optical excitation and duration of lasing oscillation.
A prism that maintains the plane of polarization may be used in place of the first Porro prism and the second Porro prism.
As described above, by using two Porro prisms arranged so that the ridgelines of the prisms are crossed at an inclined angle, each time the laser beam is reciprocated in the laser rod, it is moved on a circle concentric with the optical axis, thereby achieving uniformity of excitation distribution relative to the laser beam.
Further features of the invention, its nature and various advantages will be more apparent from the accompanying drawings and following detailed description of the invention.